1. Field of the Invention
The present invention generally relates to an inkjet head of an inkjet recording apparatus and, more particularly, to an inkjet head having a vibration plate which is deformed by an electrostatic force so as to pressurize ink stored in an ink chamber and eject a droplet of the ink from an inkjet nozzle.
2. Description of the Related Art
An inkjet printer has become popular for a personal use since the inkjet printer is smaller and less expensive than other printing apparatuses such as an electrophotographic printer.
The inkjet printer has an inkjet head which projects droplets of ink toward a recording medium such as a recording paper so as to form a dot image on the recording medium. Generally, the inkjet head is classified into three types, such as a thermal head (bubble head), a piezoelectric head and an electrostatic head, according to its principle of formation of a droplet of ink.
In the thermal head, ink is pressurized by a bubble generated by a heater provided in an ink chamber so that a droplet of ink is ejected from a inkjet nozzle by the thus-generated pressure. In the piezoelectric head, ink stored in an ink chamber is pressurized by deformation of a piezoelectric material so that a droplet of ink is ejected from an inkjet nozzle by the thus-generated pressure.
On the other hand, the electrostatic head projects a droplet of ink by deformation (warp) of a vibration plate generated by an electrostatic force. The electrostatic head can be manufactured by a fine machining technique such as a conventional semiconductor manufacturing process. Accordingly, the electrostatic head is suitable for a high-density nozzle structure having a fine structure.
In the electrostatic head, the vibration plate is formed by a semiconductor manufacturing process so that the vibration plate is formed as an electrode which is separated from another electrode with a predetermined small gap therebetween. When the electrostatic head is formed by bonding two silicon wafers, formation of such a small gap generally provides a sealed space between the vibration plate formed in one of the silicon wafers and another electrode formed in the other one of the silicon wafers. Accordingly, there is a problem in that the silicon wafers cannot be completely bonded to each other due to degassing of a material in the sealed chamber during a process for bonding the silicon wafers. That is, gasses generated by degassing may form a void in a bonding area between the silicon wafers.
It is a general object of the present invention to provide an improved and useful inkjet head in which the above-mentioned problem is eliminated.
A more specific object of the present invention is to provide an electrostatic type inkjet head which can be manufactured by a semiconductor manufacturing process without causing a problem inherent to the semiconductor manufacturing process.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an inkjet head ejecting a droplet of ink by an electrostatic force generated between a first electrode and a second electrode, the inkjet head comprising:
a first body member having a plurality of vibration plates each of which corresponds to the first electrode;
a second body member having a plurality of individual electrodes each of which corresponds to the second electrode;
spacer portions provided to at least one of the first body member and the second body member so that the first body member and the second body member are bonded to each other with the spacer portions therebetween so as to define a space providing a gap between each of the vibration plates and a respective one of the individual electrodes; and
vent passages defined by the spacer portions, the vent passages connecting each of the spaces to atmosphere.
According to the above-mentioned invention, gasses generated by degassing in the spaces providing the gaps can be released to atmosphere through the vent passages when the first body member and the second body member are bonded to each other. Thus, a uniform gap can be formed between each of the vibration plates and the respective one of the individual electrodes. This results in a uniform vibration characteristic of the vibration plates, resulting in a uniform droplet of ink being ejected from inkjet nozzles. Thus, an ink image formed by the inkjet head according to the present invention has a good image quality. Additionally, since the gap between each of the vibration plates and the respective one of the individual electrodes is uniform, it is easy to drive the vibration plates at a relatively low voltage.
Additionally, there is provided according to another aspect of the present invention an inkjet head ejecting a droplet of ink by an electrostatic force generated between a first electrode and a second electrode, the inkjet head comprising:
a first body member having a plurality of vibration plates each of which corresponds to the first electrode;
a second body member having a plurality of individual electrodes each of which corresponds to the second electrode; and
spacer portions provided to at least one of the first body member and the second body member so that the first body member and the second body member are bonded to each other with the spacer portions therebetween so as to define a space providing a gap between each of the vibration plates and a respective one of the individual electrodes,
wherein each edge of the spacer portions contacting the first body member is depressed so that the spacer portions are bonded to the first body member without edges being in contact with the first body member.
According to this invention, since the edges of the spacer portions are depressed, the spacer portions can be well bonded to the first body member if a protrusion is formed on the edges of the spacer portions during a manufacturing process of the inkjet head.
Additionally, there is provided according to another aspect of the present invention an inkjet head ejecting a droplet of ink by an electrostatic force, the inkjet head comprising:
a vibration plate defining a first electrode;
an individual electrode defining a second electrode, the individual electrode being located opposite to the vibration plate so as to generate the electrostatic force; and
protrusion patterns formed on at least one of the vibration plate and said individual electrode so that the protrusion patterns are positioned between the vibration plate and the individual electrode.
According to this invention, each of the vibration plates is prevented from sticking to the respective one of the individual electrodes or a protective film formed on each of the individual electrodes by the protrusion patterns formed on each of the individual electrodes or the protective film. The protrusion patterns can be formed as insulating layers so that the effective gap between each of the vibration plates and the respective one of the individual electrodes is decreased which facilitates movement of each of the vibration plates by the electrostatic attracting force, particularly at an initial stage of movement. This is because an electric field at the insulating layers is increased since the permittivity of the oxidation layer is about four times that of an air gap. Additionally, the protrusion patterns can be used as a stopper for the movement of each of the vibration plates when the inkjet head is rendered to be of the contact type. In such a case, the displacement of each of the vibration plates can be precisely set by the stopper by providing a relatively large drive voltage even if the drive voltage fluctuates or if the gap distance fluctuates.
Additionally, according to another aspect of the present invention, methods for manufacturing the above-mentioned inkjet heads are provided.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.